This invention relates to optical fiber communication systems and, in particular, to systems using Nd doped fiber amplifiers for amplified transmission in the 1400 nm window.
Optical fiber communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical fiber system comprises a light source, a modulator for impressing information on the light, an optical fiber transmission line for carrying the optical signals, and amplifiers for amplifying the signals along the transmission path. It also includes a receiver for detecting the signals and for demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
Optical fiber amplifiers are important components in fiber communication systems. A fiber amplifier typically comprises a length of rare earth doped fiber and a pumping source at an appropriate shorter wavelength than the transmitted signals. When the fiber amplifier is disposed in line with the transmission fiber, it amplifies transmitted signal channels passing through it. For example, Erbium doped fiber amplifiers pumped by light in the wavelength range 975-985 nm and 1470-1490 nm optically amplify signal channels in the 1530-1610 nm window of silica fiber.
New optical transmission fibers, such as All-Wave(copyright) fiber marketed by Lucent Technologies, Murray Hill, N.J. have opened up a new possible transmission window at 1400 nmxc2x150 nm. But the conventional Erbium-doped fiber amplifiers are not efficient in this wavelength range.
Raman fiber amplifiers, which work on a different principle from rare earth doped fiber amplifiers, offer one possibility for amplification in this wavelength range. But Raman amplifiers are complex and require much power.
Neodymium (Nd) doped fiber amplifiers in fluoride-based fibers were previously tried for amplification of signals within the 1300 nm window. Efficiency, however, was not good. And Nd doped fibers had a problem with amplified spontaneous emission (ASE). When praseodymium doped fibers were introduced, the work on neodymium doped fibers was abandoned.
Accordingly, there is a need for an optical fiber communication system providing efficient fiber amplification of signal channels transmitted in the 1400 nm window.
In accordance with the invention, an optical fiber communication system comprises one or more Nd doped fiber amplifiers for amplified transmission in the 1400 nm window. The amplifier is designed with a combination of waveguide effects and selective absorption to reduce amplified spontaneous emission to acceptable levels.